Electromagnetic filters commonly use various dielectric materials in resonators in order to filter unwanted frequencies from an input signal. By loading, or placing a conductor in or adjacent to the dielectric material, the size and thus the cost of such components can be reduced. Because of higher resistance, the use of ordinary conductors will result in significant electromagnetic losses in the component. Superconducting materials have therefore been substituted for the ordinary conductors because of their extremely low surface resistance, and thus low loss.
The use of superconducting materials results in other complications for manufacturing such devices. First, superconducting materials must be cooled to a temperature at or below their critical temperatures in order to have the desirable low surface resistance. Second, in order for superconducting materials to have a significant benefit, the dielectric material used in conjunction with those superconducting materials must have a low loss tangent. The loss tangent of a dielectric material is defined as the ratio of the imaginary term in its permitivity, .di-elect cons.*, to the real term in its permitivity .di-elect cons..sub.r, or tan .delta.=.di-elect cons.*/.di-elect cons..sub.r. It is manifest as a material property in the form of the Q of a resonator made from the dielectric. If a piece of the dielectric is suspended in free space and allowed to resonate, the quality factor Q, of such a resonator will be Q=1/tan .delta.. Thus, loss tangent may be measured by placing a sample of the material on a polytetrafluoroethylene (virtually invisible to RF) pedestal and measuring its Q factor. Since a superconducting device will operate at cryogenic temperatures, the dielectric material must exhibit such a low loss tangent at those temperatures.
In most materials, the loss tangent of a dielectric material will decrease as the temperature of that dielectric material decreases. See Shield, T. C. et al., "Thick Films of YBCO on Alumina Substrates with Zirconia Barrier Layers," Supercond. Sci. Technol. 5 (1992). However, a dielectric material which exhibits a relatively low loss tangent at room temperature may not have a relatively low loss tangent at cryogenic temperatures.